is an eXtreme sport.
is an eXperimental plane (X-plane)
Is a mutant with eXtra ordinary powers (X-men)
Is full of eXcitement and fun
Is a big eXplosion
The X is out there (X Files)X marks the spot (Indiana Jones)

Linda and the Diatonic Scale
Music is love in search of a word.
- French novelist and performer Sidonie Gabrielle
Dearest Linda:
As the moment of my departure on the ship that will take me away and separate us draws near, I feel compelled to write you something meaningful...

To me, "X" means underlying is something special. it could be buried treasure. Possibly, it is just a dirty stick or twig. It still makes it wonderful and special. Maybe not so much, since it is not too awful useful, but it could be a clue or...